BUG: Disable bottleneck for nanops to avoid overflow

Author: Javier Sorribes Camargo

pandas/core/nanops.py

@@ -129,10 +129,7 @@ def _bn_ok_dtype(dt, name):
         # bottleneck does not properly upcast during the sum
         # so can overflow
         if name == 'nansum':
-            if dt.itemsize < 8:
-                return False
-            elif os.name == 'nt':  # bottleneck has overflow issue in Windows
-                return False
+            return False
 
         return True
     return False